Illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offers an efficient and long-lived alternative to fluorescent, high-intensity discharge and traditional incandescent lamps. Many LED light sources employ high powered LEDs, which pose thermal management problems and other related problems. Another drawback with state of the art LED devices is a high initial cost.
Currently, gallium nitride (GaN) based LEDs are epitaxially grown on sapphire substrates. These substrates have disadvantages such as high cost, low thermal conductivity at temperatures of interest and they are electrical insulators. Another problem with sapphire as a substrate is its chemical inertness, which makes it difficult to release epitaxially grown material using a chemical etching process.
In an ideal situation, LEDs would be grown on cheap, thermally and electrically conductive substrates, such as silicon. However, growing GaN material on a silicon substrate provides significant challenges due to the significant lattice mismatch between silicon and GaN as well as too high a difference between the coefficients of thermal expansion of the two materials. This results in high defect densities of the materials grown as well as poor performance. There are several approaches common in the industry to mitigate the problems. In one approach complex buffer layers are grown to compensate for thermal and lattice mismatch between the substrate and the epitaxial layer. In a different solution an epitaxial lateral-overgrowth (ELOG) process is chosen. In yet a different solution the epitaxial layer is grown on islands considerably smaller than the entire wafer.
There are certain advantages of using micro-LEDs in lighting devices. Currently, the only feasible process to transfer semiconductor die including micro-LEDs is to use an elastomeric stamping process. In order to allow for such a stamping transfer process, the semiconductor material needs to be specially prepared and processed. Firstly, a sacrificial layer is required on the substrate or in the epitaxial stack to allow for the release of the semiconductor die. Secondly, the epitaxial layer needs to be specially processed to provide anchoring for the semiconductor die during and after the release process. Thirdly, the sacrificial layer is removed (e.g. through a wet etching process), leaving an array of semiconductor die suspended above the substrate ready for the transfer process to take place. FIG. 1A shows a prior art process in which an epitaxial layer 14 is grown above a sacrificial layer 12 on a substrate 10. Referring to FIG. 1B, after preparation of an anchor structure 16 etching of the sacrificial layer 12 leaves a gap 18 between substrate 10 and suspended semiconductor die 14A formed from the epitaxial layer 14.
Another process is to transfer epitaxially grown material from a sapphire substrate to a silicon substrate via wafer bonding and subsequent removal of the sapphire using laser lift off. The silicon still needs to be prepared for release etching of the functional epitaxial material. This process, however, introduces additional process steps and further expense.
There is still, therefore, a need for an alternative method of manufacturing transferable semiconductor die which is less complex and costly.